Electron Energization and Radiation in MRI-driven Accretion Edison Liang, Rice University Collaborators: G. Hilburn, Rice; S. Liu, H. Li, LANL; C. Gammie, UI; M. Boettcher, Ohio U. DPP talk November 2008

High-energy emission of LLBH such as SgrA* examplifies accretion which requires electron energization above the level predicted by e-ion coulomb coupling flare quiescent (from S. Liu et al )

Motivation In low-luminosity black holes (LLBH) such as Sgr A*, Coulomb heating by ions is too inefficient due to low density Can MRI-driven turbulence directly heat relativistic electrons? We find that wave turbulence alone provides only modest heating. But anomalous heating by thin current sheets, leads to a much hotter superthermal component.

weakly magnetized initial torus MRI-induced accretion flow with saturated MHD turbulence thermal MRI disk models new approach compressional heating of ions turbulence energization of nonthermal electrons and ions coulomb heating of electrons by virial ions synchrotron emission by nonthermal electrons thermal cyclotron emission at low energy SSC+EC of nonthermal electrons SSC + EC emission at high energy pion decay emission of Nonthermal ions

512x512 HARM GRMHD Run B2 

current sheets get thinner with increasing resolution but pattern maintains self-similarity B2 512x512 HARM code runs 256x256

folded current sheets are a dominant feature of MRI-driven turbulence Br B | J |

2.5 D PIC 1024x1024 doubly periodic grid, ~108 particles, mi=100me Bx,y=Bosink(y,x) Bz in Jx Ti=0.25mec2 Te=0.25mec2 or 1.5mec2 ywe/c Bz out Jx Bz in xwe/c

current sheet thickens and bends due to wave perturbations single mode kL=4p Te=1.5mec2 Bz=10Bo We=5w pe current sheet thickens and bends due to wave perturbations twe=0 1000 4000 Bz

Bx twe=1000 4000 By

Jx twe=1000 4000 Jz

Magnetic energy is efficiently converted to hot electrons due to enhanced reconnection Eem EBz Eparticle Ee Eion EE twe twe EBxy

Electrons are heated to form a 2-component Maxwellian: Lower-T component heated by Alfven wave cascade Higher-T component heated by current sheet dissipation fe(g) Te1~2MeV Te2~10MeV g g twe=4000 Results are in agreement with those of Zenitani and Hoshino (2005)

ions are also heated, likely by electrostatic modes fi(g) 4000 twe=1000 ion

No CS fe(g) CS fe(g) Te=0.25mec2 g g fe(g) No CS CS fe(g) Te=1.5mec2 g g

MC photon transport

HARM to MC grid The HARM code's grid is divided logarithmically in r and concentrated to the equatorial plane The MC grid is divided equally in r and in z Overlaying these two invariably leads to under- or oversampling in areas

Photon spectrum using HARM output as input for the 2D-MC code (95x95 grid) with density normalized by the Chandra flare as due to bremsstrahlung. Note that our Compton hump is lower than the result of Ohsuga et al 2005. synchrotron Compton bremsstrahlung Ohsuga et al 2005

Summary Many LLBH exhibit superthermal/nonthermal spectra that require anomalous heating of electrons. We explore such energization using MHD turbulence self-generated in MRI - induced accretion flows. PIC simulations suggest that current sheet dissipation is the dominant heating mechanism, producing a 2-component electron spectrum. It would be very interesting to see if the x-ray spectra during flares and quiescence can be explained by current sheet heating of electrons.

Te=0.25mec2 gives roughly the same results: 2/3 electrons, 1/3 ions Above results seem to be insensitive to the initial electron temperature: Te=0.25mec2 gives roughly the same results: 2/3 electrons, 1/3 ions twe twe

Initial single mode cascades into higher and higher modes via parametric conversion By Bx x y